Unraveling microenvironment modification in an atomically dispersed bimetallic FeCu catalyst in the oxygen reduction reaction†

The important effect of microstructure in Fe–Cu bimetallic catalysts on the mechanism of the oxygen reduction reaction (ORR) was theoretically and experimentally investigated. Three types of Fe sites regulated by Cu were constructed: Fe clusters modified with Cu–N₄, Fe–Cu dimers, and isolated Fe/Cu single atoms. A theoretical study revealed that although copper could reduce the d-band center of Fe, the Fe–Cu dimer displayed unique attributes. Notably, the dimer increased the energy of *π antibonding orbitals combined with Fe²⁺/Fe³⁺-3d and *OH-2p, accelerated *OH removal and produced the lowest predicted overpotential (0.48 V). Three catalysts featuring the above models were experimentally embedded on porous nitrogen-doped carbon. FeCu-NC-2 with Fe–Cu dimers exhibited the most positive half-wave potentials of 0.904 V in alkaline and 0.720 V in neutral solutions. A Zn–air battery and a microbial fuel cell equipped with FeCu-NC-2 as the cathodic catalyst produced stable and high power densities of 568.6 mW cm⁻² and 2467 mW m⁻², respectively.